The present invention relates to a flatness detector for detecting flatness of a strip such as rolled steel.
In rolling a metal plate or the like into a strip, there has been employed a control system in which tension distribution along width of the strip is measured. Based on the measured distribution, flatness of the strip is detected and control is made to improve the flatness.
A conventional flatness detector comprises a fixed shaft and a plurality of rotary rings loosely fitted over the shaft and adjacent to each other along an axis of the shaft, compressed air being supplied to a gap between the rings and the shaft to thereby provide a pneumatic bearing. With this flatness detector, any variation of the gap is detected in terms of pressure variations which in turn are used to detect the flatness of the, strip.
With the conventional flatness detector, the compressed air supplied to the pneumatic bearing is usually dry air so that the shaft and the rings fitted thereover are super cooled by the dry air whereas the moving rolled strip is hot. As a result, when the hot strip contacts the supercooled rings to receive sudden temperature variations, lengthwise wrinkles are produced on the strip due to shrinkage of the strip.
Such lengthwise wrinkles on the strip may be prevented by increasing tension of the strip upon rolling. However, this would be disadvantageous in that the strip being rolled tends to be torn, resulting in lowering of productivity.
In order to prevent the lengthwise wrinkles, it may be also envisaged or contemplated that a flatness detector is finished to have a crowned contour so as to vary the tension imparted to the strip along the width of the strip and suppress production of the lengthwise wrinkles on the strip. This idea has been already applied to conventional single rolls.
However, it is extremely difficult to make crowned a flatness detector having a plurality of mutually adjacent rotary rings as mentioned above.
The present invention was made to overcome the above problems encountered in the prior art and has for its object to provide a flatness detector which can substantially eliminate production of lengthwise wrinkles on a strip.